C is a programming language of many different dialects, similar to the way that each spoken language has many different dialects. In C, dialects don't exist because the speakers live in the North or South. Instead, they're there because there are many different compilers that support slightly different features. There are several common compilers: in particular, Borland C++, Microsoft C++, and GNU C. There are also many front-end environments for the different compilers--the most common is Dev-C++ around GNU's G++ compiler. Some, such as GCC, are free, while others are not. Please see the compiler listing for more information on how to get a compiler and set it up. You should note that if you are programming in C on a C++ compiler, then you will want to make sure that your compiler attempts to compile C instead of C++ to avoid small compatability issues in later tutorials. Each of these compilers is slightly different. Each one should support the ANSI standard C functions, but each compiler will also have nonstandard functions (these functions are similar to slang spoken in different parts of a country). Sometimes the use of nonstandard functions will cause problems when you attempt to compile source code (the actual C code written by a programmer and saved as a text file) with a different compiler. These tutorials use ANSI standard C and should not suffer from this problem; fortunately, since C has been around for quite a while, there shouldn't be too many compatability issues except when your compiler tries to create C++ code. If you don't yet have a compiler, I strongly recommend finding one now. A simple compiler is sufficient for our use, but make sure that you do get one in order to get the most from these tutorials. The page linked above, compilers, lists compilers by operating system. Every full C program begins inside a function called "main". A function is simply a collection of commands that do "something". The main function is always called when the program first executes. From main, we can call other functions, whether they be written by us or by others or use built-in language features. To access the standard functions that comes with your compiler, you need to include a header with the #include directive. What this does is effectively take everything in the header and paste it into your program. Let's look at a working program: #include
int main()
{
printf( "I am alive! Beware.\n" );
getchar();
return 0;
}
Let's look at the elements of the program. The #include is a "preprocessor" directive that tells the compiler to put code from the header called stdio.h into our program before actually creating the executable. By including header files, you an gain access to many different functions. For example, the printf function requires getchar. The semicolon is part of the syntax of C. It tells the compiler that you're at the end of a command. You will see later that the semicolon is used to end most commands in C. The next imporant line is int main(). This line tells the compiler that there is a function named main, and that the function returns an integer, hence int. The "curly braces" ({ and }) signal the beginning and end of functions and other code blocks. If you have programmed in Pascal, you will know them as BEGIN and END. Even if you haven't programmed in Pascal, this is a good way to think about their meaning. The printf function is the standard C way of displaying output on the screen. The quotes tell the compiler that you want to output the literal string as-is (almost). The '\n' sequence is actually treated as a single character that stands for a newline (we'll talk about this later in more detail); for the time being, just remember that there are a few sequences that, when they appear in a string literal, are actually not displayed literally by printf and that '\n' is one of them. The actual effect of '\n' is to move the cursor on your screen to the next line. Again, notice the semicolon: it is added onto the end of all lines, such as function calls, in C. The next command is getchar(). This is another function call: it reads in a single character and waits for the user to hit enter before reading the character. This line is included because many compiler environments will open a new console window, run the program, and then close the window before you can see the output. This command keeps that window from closing because the program is not done yet because it waits for you to hit enter. Including that line gives you time to see the program run. Finally, at the end of the program, we return a value from main to the operating system by using the return statement. This return value is important as it can be used to tell the operating system whether our program succeeded or not. A return value of 0 means success. The final brace closes off the function. You should try compiling this program and running it. You can cut and paste the code into a file, save it as a .c file, and then compile it. If you are using a command-line compiler, such as Borland C++ 5.5, you should read the compiler instructions for information on how to compile. Otherwise compiling and running should be as simple as clicking a button with your mouse (perhaps the "build" or "run" button). You might start playing around with the printf function and get used to writing simple C programs.
Explaining your CodeComments are critical for all but the most trivial programs and this tutorial will often use them to explain sections of code. When you tell the compiler a section of text is a comment, it will ignore it when running the code, allowing you to use any text you want to describe the real code. To create a comment in C, you surround the text with /* and then */ to block off everything between as a comment. Certain compiler environments or text editors will change the color of a commented area to make it easier to spot, but some will not. Be certain not to accidentally comment out code (that is, to tell the compiler part of your code is a comment) you need for the program. When you are learning to program, it is also useful to comment out sections of code in order to see how the output is affected.
Using VariablesSo far you should be able to write a simple program to display information typed in by you, the programmer and to describe your program with comments. That's great, but what about interacting with your user? Fortunately, it is also possible for your program to accept input. But first, before you try to receive input, you must have a place to store that input. In programming, input and data are stored in variables. There are several different types of variables; when you tell the compiler you are declaring a variable, you must include the data type along with the name of the variable. Several basic types include char, int, and float. Each type can store different types of data. A variable of type char stores a single character, variables of type int store integers (numbers without decimal places), and variables of type float store numbers with decimal places. Each of these variable types - char, int, and float - is each a keyword that you use when you declare a variable. Some variables also use more of the computer's memory to store their values. It may seem strange to have multiple variable types when it seems like some variable types are redundant. But using the right variable size can be important for making your program efficient because some variables require more memory than others. For now, suffice it to say that the different variable types will almost all be used! Before you can use a variable, you must tell the compiler about it by declaring it and telling the compiler about what its "type" is. To declare a variable you use the syntax
int myVariable;
Note once again the use of a semicolon at the end of the line. Even though we're not calling a function, a semicolon is still required at the end of the "expression". This code would create a variable called myVariable; now we are free to use myVariable later in the program. It is permissible to declare multiple variables of the same type on the same line; each one should be separated by a comma. If you attempt to use an undefined variable, your program will not run, and you will receive an error message informing you that you have made a mistake. Here are some variable declaration examples:
int x;
int a, b, c, d;
char letter;
float the_float;
While you can have multiple variables of the same type, you cannot have multiple variables with the same name. Moreover, you cannot have variables and functions with the same name. A final restriction on variables is that variable declarations must come before other types of statements in the given "code block" (a code block is just a segment of code surrounded by { and }). So in C you must declare all of your variables before you do anything else: Wrong
#include
int main()
{
/* wrong! The variable declaration must appear first */
printf( "Declare x next" );
int x;
return 0;
}
Fixed
#include
{
int x;
printf( "Declare x first" );
return 0;
}
Reading inputUsing variables in C for input or output can be a bit of a hassle at first, but bear with it and it will make sense. We'll be using the scanf function to read in a value and then printf to read it back out. Let's look at the program and then pick apart exactly what's going on. You can even compile this and run it if it helps you follow along.
#include
int main()
{
int this_is_a_number;
printf( "Please enter a number: " );
scanf( "%d", &this_is_a_number );
printf( "You entered %d", this_is_a_number );
getchar();
}
So what does all of this mean? We've seen the #include and main function before; main must appear in every program you intend to run, and the #include gives us access to printf (as well as scanf). (As you might have guessed, the io in stdio.h stands for "input/output"; std just stands for "standard.") The keyword int declares this_is_a_number to be an integer. This is where things start to get interesting: the scanf function works by taking a string and some variables modified with &. The string tells scanf what variables to look for: notice that we have a string containing only "%d" -- this tells the scanf function to read in an integer. The second argument of scanf is the variable, sort of. We'll learn more about what is going on later, but the gist of it is that scanf needs to know where the variable is stored in order to change its value. Using & in front of a variable allows you to get its location and give that to scanf instead of the value of the variable. Think of it like giving someone directions to the soda aisle and letting them go get a coca-cola instead of fetching the coke for that person. The & gives the scanf function directions to the variable. When the program runs, each call to scanf checks its own input string to see what kinds of input to expect, and then stores the value input into the variable. The second printf statement also contains the same '%d'--both scanf and printf use the same format for indicating values embedded in strings. In this case, printf takes the first argument after the string, the variable this_is_a_number, and treats it as though it were of the type specified by the "format specifier". In this case, printf treats this_is_a_number as an integer based on the format specifier. So what does it mean to treat a number as an integer? If the user attempts to type in a decimal number, it will be truncated (that is, the decimal component of the number will be ignored) when stored in the variable. Try typing in a sequence of characters or a decimal number when you run the example program; the response will vary from input to input, but in no case is it particularly pretty. Of course, no matter what type you use, variables are uninteresting without the ability to modify them. Several operators used with variables include the following: *, -, +, /, =, ==, >, <. The * multiplies, the / divides, the - subtracts, and the + adds. It is of course important to realize that to modify the value of a variable inside the program it is rather important to use the equal sign. In some languages, the equal sign compares the value of the left and right values, but in C == is used for that task. The equal sign is still extremely useful. It sets the value of the variable on the left side of the equals sign equal to the value on the right side of the equals sign. The operators that perform mathematical functions should be used on the right side of an equal sign in order to assign the result to a variable on the left side. Here are a few examples: a = 4 * 6; /* (Note use of comments and of semicolon) a is 24 */ a = a + 5; /* a equals the original value of a with five added to it */ a == 5 /* Does NOT assign five to a. Rather, it checks to see if a equals 5.*/ The other form of equal, ==, is not a way to assign a value to a variable. Rather, it checks to see if the variables are equal. It is extremely useful in many areas of C; for example, you will often use == in such constructions as conditional statements and loops. You can probably guess how <> function. They are greater than and less than operators. For example:
a <> 5 /* Checks to see if a is greater than five */
a == 5 /* Checks to see if a equals five, for good measure */
Lesson 2: If statements
The ability to control the flow of your program, letting it make
decisions on what code to execute, is valuable to the programmer. The
if statement allows you to control if a program enters a section of
code or not based on whether a given condition is true or false. One of
the important functions of the if statement is that it allows the
program to select an action based upon the user's input. For example,
by using an if statement to check a user-entered password, your program
can decide whether a user is allowed access to the program.
Without a conditional statement such as the if statement, programs
would run almost the exact same way every time, always following the
same sequence of function calls. If statements allow the flow of the
program to be changed, which leads to more interesting code.
Before discussing the actual structure of the if statement, let us
examine the meaning of TRUE and FALSE in computer terminology. A true
statement is one that evaluates to a nonzero number. A false statement
evaluates to zero. When you perform comparison with the relational
operators, the operator will return 1 if the comparison is true, or 0
if the comparison is false. For example, the check 0 == 2 evaluates to
0. The check 2 == 2 evaluates to a 1. If this confuses you, try to use
a printf statement to output the result of those various comparisons
(for example printf ( "%d", 2 == 1 );)
When programming, the aim of the program will often require the
checking of one value stored by a variable against another value to
determine whether one is larger, smaller, or equal to the other.
There are a number of operators that allow these checks.
Here are the relational operators, as they are known, along with
examples: > greater than 5 > 4 is TRUE<>= greater than or equal 4 >= 4 is TRUE<= less than or equal 3 <= 4 is TRUE== equal to 5 == 5 is TRUE!= not equal to 5 != 4 is TRUE
It is highly probable that you have seen these before, probably with
slightly different symbols. They should not present any hindrance to
understanding. Now that you understand TRUE and FALSE well as the
comparison operators, let us look at the actual structure of if
statements.
The structure of an if statement is as follows: if ( statement is TRUE ) Execute this line of code
Here is a simple example that shows the syntax: if ( 5 < 10 ) printf( "Five is now less than ten, that's a big surprise" );
Here, we're just evaluating the statement, "is five less than ten", to
see if it is true or not; with any luck, it's not! If you want, you can
write your own full program including stdio.h and put this in the main
function and run it to test.
To have more than one statement execute after an if statement that
evaluates to true, use braces, like we did with the body of the main
function. Anything inside braces is called a compound statement, or a
block. When using if statements, the code that depends on the if
statement is called the "body" of the if statement.
For example: if ( TRUE ) { /* between the braces is the body of the if statement */ Execute all statements inside the body}
I recommend always putting braces following if statements. If you do
this, you never have to remember to put them in when you want more than
one statement to be executed, and you make the body of the if statement
more visually clear.
ElseSometimes when the condition in an if statement evaluates to false, it
would be nice to execute some code instead of the code executed when
the statement evalutes to true. The "else" statement effectively says
that whatever code after it (whether a single line or code between
brackets) is executed if the if statement is FALSE.
It can look like this: if ( TRUE ) { /* Execute these statements if TRUE */}else { /* Execute these statements if FALSE */}
Else ifAnother use of else is when there are multiple conditional statements
that may all evaluate to true, yet you want only one if statement's
body to execute. You can use an "else if" statement following an if
statement and its body; that way, if the first statement is true, the
"else if" will be ignored, but if the if statement is false, it will
then check the condition for the else if statement. If the if statement
was true the else statement will not be checked. It is possible to use
numerous else if statements to ensure that only one block of code is
executed.
Let's look at a simple program for you to try out on your own. #include
int main() /* Most important part of the
program!*/{ int age; /* Need a variable... */ printf( "Please enter your age" ); /* Asks for age */ scanf( "%d", &age ); /* The input is put in age */ if ( age < 100 ) { /* If the age is less than 100
*/ printf ("You are pretty young!\n" ); /* Just to show you it
works... */ } else if ( age == 100 ) { /* I use else just to show an
example */ printf( "You are old\n" ); } else { printf( "You are really old\n" ); /* Executed if no other
statement is */ } return 0;}
More interesting conditions using boolean operatorsBoolean operators allow you to create more complex conditional
statements. For example, if you wish to check if a variable is both
greater than five and less than ten, you could use the Boolean AND to
ensure both var > 5 and var < 10 are true. In the following discussion
of Boolean operators, I will capitalize the Boolean operators in order
to distinguish them from normal English. The actual C operators of
equivalent function will be described further along into the tutorial -
the C symbols are not: OR, AND, NOT, although they are of equivalent
function.
When using if statements, you will often wish to check multiple
different conditions. You must understand the Boolean operators OR,
NOT, and AND. The boolean operators function in a similar way to the
comparison operators: each returns 0 if evaluates to FALSE or 1 if it
evaluates to TRUE.
NOT: The NOT operator accepts one input. If that input is TRUE, it
returns FALSE, and if that input is FALSE, it returns TRUE. For
example, NOT (1) evalutes to 0, and NOT (0) evalutes to 1. NOT (any
number but zero) evaluates to 0. In C NOT is written as !. NOT is
evaluated prior to both AND and OR.
AND: This is another important command. AND returns TRUE if both inputs
are TRUE (if 'this' AND 'that' are true). (1) AND (0) would evaluate to
zero because one of the inputs is false (both must be TRUE for it to
evaluate to TRUE). (1) AND (1) evaluates to 1. (any number but 0) AND
(0) evaluates to 0. The AND operator is written && in C. Do not be
confused by thinking it checks equality between numbers: it does not.
Keep in mind that the AND operator is evaluated before the OR operator.
OR: Very useful is the OR statement! If either (or both) of the two
values it checks are TRUE then it returns TRUE. For example, (1) OR (0)
evaluates to 1. (0) OR (0) evaluates to 0. The OR is written as in
C. Those are the pipe characters. On your keyboard, they may look like
a stretched colon. On my computer the pipe shares its key with \. Keep
in mind that OR will be evaluated after AND.
It is possible to combine several Boolean operators in a single
statement; often you will find doing so to be of great value when
creating complex expressions for if statements. What is !(1 && 0)? Of
course, it would be TRUE. It is true is because 1 && 0 evaluates to 0
and !0 evaluates to TRUE (ie, 1).
Try some of these - they're not too hard. If you have questions about
them, feel free to stop by our forums. A. !( 1 0 ) ANSWER: 0 B. !( 1 1 && 0 ) ANSWER: 0 (AND is evaluated before OR)C. !( ( 1 0 ) && 0 ) ANSWER: 1 (Parenthesis are useful)
Lesson 3: LoopsLoops are used to repeat a block of code. Being able to have your
program repeatedly execute a block of code is one of the most basic but
useful tasks in programming -- many programs or websites that produce
extremely complex output (such as a message board) are really only
executing a single task many times. (They may be executing a small
number of tasks, but in principle, to produce a list of messages only
requires repeating the operation of reading in some data and displaying
it.) Now, think about what this means: a loop lets you write a very
simple statement to produce a significantly greater result simply by
repetition.
One caveat: before going further, you should understand the concept of
C's true and false, because it will be necessary when working with
loops (the conditions are the same as with if statements). This concept
is covered in the previous tutorial. There are three types of loops:
for, while, and do..while. Each of them has their specific uses. They
are all outlined below.
FOR - for loops are the most useful type. The syntax for a for loop is
for ( variable initialization; condition; variable update ) { Code to execute while the condition is true}
The variable initialization allows you to either declare a variable and
give it a value or give a value to an already existing variable.
Second, the condition tells the program that while the conditional
expression is true the loop should continue to repeat itself. The
variable update section is the easiest way for a for loop to handle
changing of the variable. It is possible to do things like x++, x = x +
10, or even x = random ( 5 ), and if you really wanted to, you could
call other functions that do nothing to the variable but still have a
useful effect on the code. Notice that a semicolon separates each of
these sections, that is important. Also note that every single one of
the sections may be empty, though the semicolons still have to be
there. If the condition is empty, it is evaluated as true and the loop
will repeat until something else stops it.
Example: #include
int main(){ int x; /* The loop goes while x < 10, and x increases by one every loop*/ for ( x = 0; x < 10; x++ ) { /* Keep in mind that the loop condition checks the conditional statement before it loops again. consequently, when x equals 10 the loop breaks. x is updated before the condition is checked. */ printf( "%d\n", x ); } getchar();}
This program is a very simple example of a for loop. x is set to zero,
while x is less than 10 it calls printf to display the value of the
variable x, and it adds 1 to x until the condition is met. Keep in mind
also that the variable is incremented after the code in the loop is run
for the first time.
WHILE - WHILE loops are very simple. The basic structure is
while ( condition ) { Code to execute while the condition is true }
The true represents a boolean expression which could be x == 1 or while
( x != 7 ) (x does not equal 7). It can be any combination of boolean
statements that are legal. Even, (while x ==5 v == 7) which says
execute the code while x equals five or while v equals 7. Notice that a
while loop is like a stripped-down version of a for loop-- it has no
initialization or update section. However, an empty condition is not
legal for a while loop as it is with a for loop.
Example: #include
int main(){ int x = 0; /* Don't forget to declare variables */ while ( x < 10 ) { /* While x is less than 10 */ printf( "%d\n", x ); x++; /* Update x so the condition can be met
eventually */ } getchar();}
This was another simple example, but it is longer than the above FOR
loop. The easiest way to think of the loop is that when it reaches the
brace at the end it jumps back up to the beginning of the loop, which
checks the condition again and decides whether to repeat the block
another time, or stop and move to the next statement after the block.
DO..WHILE - DO..WHILE loops are useful for things that want to loop at
least once. The structure is do {} while ( condition );
Notice that the condition is tested at the end of the block instead of
the beginning, so the block will be executed at least once. If the
condition is true, we jump back to the beginning of the block and
execute it again. A do..while loop is almost the same as a while loop
except that the loop body is guaranteed to execute at least once. A
while loop says "Loop while the condition is true, and execute this
block of code", a do..while loop says "Execute this block of code, and
then continue to loop while the condition is true".
Example: #include
int main(){ int x;
x = 0; do { /* "Hello, world!" is printed at least one time even though the condition is false*/ printf( "%d\n", x ); } while ( x != 0 ); getchar();}
Keep in mind that you must include a trailing semi-colon after the
while in the above example. A common error is to forget that a
do..while loop must be terminated with a semicolon (the other loops
should not be terminated with a semicolon, adding to the confusion).
Notice that this loop will execute once, because it automatically
executes before checking the condition. Break and Continue
Two keywords that are very important to looping are break and continue.
The break command will exit the most immediately surrounding loop
regardless of what the conditions of the loop are. Break is useful if
we want to exit a loop under special circumstances. For example, let's
say the program we're working on is a two-person checkers game. The
basic structure of the program might look like this: while (true) { take_turn(player1); take_turn(player2);}This will make the game alternate between having player 1 and player 2
take turns. The only problem with this logic is that there's no way to
exit the game; the loop will run forever! Let's try something like this
instead: while(true){ if (someone_has_won() someone_wants_to_quit() == TRUE) {break;} take_turn(player1); if (someone_has_won() someone_wants_to_quit() == TRUE) {break;} take_turn(player2);}This code accomplishes what we want--the primary loop of the game will
continue under normal circumstances, but under a special condition
(winning or exiting) the flow will stop and our program will do
something else.Continue is another keyword that controls the flow of loops. If you are
executing a loop and hit a continue statement, the loop will stop its
current iteration, update itself (in the case of for loops) and begin
to execute again from the top. Essentially, the continue statement is
saying "this iteration of the loop is done, let's continue with the
loop without executing whatever code comes after me." Let's say we're
implementing a game of Monopoly. Like above, we want to use a loop to
control whose turn it is, but controlling turns is a bit more
complicated in Monopoly than in checkers. The basic structure of our
code might then look something like this: for (player = 1; someone_has_won == FALSE; player++) { if (player > total_number_of_players) {player = 1;} if (is_bankrupt(player)) {continue;} take_turn(player); }This way, if one player can't take her turn, the game doesn't stop for
everybody; we just skip her and keep going with the next player's turn.
Lesson 3: LoopsLoops are used to repeat a block of code. Being able to have your
program repeatedly execute a block of code is one of the most basic but
useful tasks in programming -- many programs or websites that produce
extremely complex output (such as a message board) are really only
executing a single task many times. (They may be executing a small
number of tasks, but in principle, to produce a list of messages only
requires repeating the operation of reading in some data and displaying
it.) Now, think about what this means: a loop lets you write a very
simple statement to produce a significantly greater result simply by
repetition.
One caveat: before going further, you should understand the concept of
C's true and false, because it will be necessary when working with
loops (the conditions are the same as with if statements). This concept
is covered in the previous tutorial. There are three types of loops:
for, while, and do..while. Each of them has their specific uses. They
are all outlined below.
FOR - for loops are the most useful type. The syntax for a for loop is
for ( variable initialization; condition; variable update ) { Code to execute while the condition is true}
The variable initialization allows you to either declare a variable and
give it a value or give a value to an already existing variable.
Second, the condition tells the program that while the conditional
expression is true the loop should continue to repeat itself. The
variable update section is the easiest way for a for loop to handle
changing of the variable. It is possible to do things like x++, x = x +
10, or even x = random ( 5 ), and if you really wanted to, you could
call other functions that do nothing to the variable but still have a
useful effect on the code. Notice that a semicolon separates each of
these sections, that is important. Also note that every single one of
the sections may be empty, though the semicolons still have to be
there. If the condition is empty, it is evaluated as true and the loop
will repeat until something else stops it.
Example: #include
int main(){ int x; /* The loop goes while x < 10, and x increases by one every loop*/ for ( x = 0; x < 10; x++ ) { /* Keep in mind that the loop condition checks the conditional statement before it loops again. consequently, when x equals 10 the loop breaks. x is updated before the condition is checked. */ printf( "%d\n", x ); } getchar();}
This program is a very simple example of a for loop. x is set to zero,
while x is less than 10 it calls printf to display the value of the
variable x, and it adds 1 to x until the condition is met. Keep in mind
also that the variable is incremented after the code in the loop is run
for the first time.
WHILE - WHILE loops are very simple. The basic structure is
while ( condition ) { Code to execute while the condition is true }
The true represents a boolean expression which could be x == 1 or while
( x != 7 ) (x does not equal 7). It can be any combination of boolean
statements that are legal. Even, (while x ==5 v == 7) which says
execute the code while x equals five or while v equals 7. Notice that a
while loop is like a stripped-down version of a for loop-- it has no
initialization or update section. However, an empty condition is not
legal for a while loop as it is with a for loop.
Example: #include
int main(){ int x = 0; /* Don't forget to declare variables */ while ( x < 10 ) { /* While x is less than 10 */ printf( "%d\n", x ); x++; /* Update x so the condition can be met
eventually */ } getchar();}
This was another simple example, but it is longer than the above FOR
loop. The easiest way to think of the loop is that when it reaches the
brace at the end it jumps back up to the beginning of the loop, which
checks the condition again and decides whether to repeat the block
another time, or stop and move to the next statement after the block.
DO..WHILE - DO..WHILE loops are useful for things that want to loop at
least once. The structure is do {} while ( condition );
Notice that the condition is tested at the end of the block instead of
the beginning, so the block will be executed at least once. If the
condition is true, we jump back to the beginning of the block and
execute it again. A do..while loop is almost the same as a while loop
except that the loop body is guaranteed to execute at least once. A
while loop says "Loop while the condition is true, and execute this
block of code", a do..while loop says "Execute this block of code, and
then continue to loop while the condition is true".
Example: #include
int main(){ int x;
x = 0; do { /* "Hello, world!" is printed at least one time even though the condition is false*/ printf( "%d\n", x ); } while ( x != 0 ); getchar();}
Keep in mind that you must include a trailing semi-colon after the
while in the above example. A common error is to forget that a
do..while loop must be terminated with a semicolon (the other loops
should not be terminated with a semicolon, adding to the confusion).
Notice that this loop will execute once, because it automatically
executes before checking the condition. Break and Continue
Two keywords that are very important to looping are break and continue.
The break command will exit the most immediately surrounding loop
regardless of what the conditions of the loop are. Break is useful if
we want to exit a loop under special circumstances. For example, let's
say the program we're working on is a two-person checkers game. The
basic structure of the program might look like this: while (true) { take_turn(player1); take_turn(player2);}This will make the game alternate between having player 1 and player 2
take turns. The only problem with this logic is that there's no way to
exit the game; the loop will run forever! Let's try something like this
instead: while(true){ if (someone_has_won() someone_wants_to_quit() == TRUE) {break;} take_turn(player1); if (someone_has_won() someone_wants_to_quit() == TRUE) {break;} take_turn(player2);}This code accomplishes what we want--the primary loop of the game will
continue under normal circumstances, but under a special condition
(winning or exiting) the flow will stop and our program will do
something else.Continue is another keyword that controls the flow of loops. If you are
executing a loop and hit a continue statement, the loop will stop its
current iteration, update itself (in the case of for loops) and begin
to execute again from the top. Essentially, the continue statement is
saying "this iteration of the loop is done, let's continue with the
loop without executing whatever code comes after me." Let's say we're
implementing a game of Monopoly. Like above, we want to use a loop to
control whose turn it is, but controlling turns is a bit more
complicated in Monopoly than in checkers. The basic structure of our
code might then look something like this: for (player = 1; someone_has_won == FALSE; player++) { if (player > total_number_of_players) {player = 1;} if (is_bankrupt(player)) {continue;} take_turn(player); }This way, if one player can't take her turn, the game doesn't stop for
everybody; we just skip her and keep going with the next player's turn.
Lesson 4: FunctionsNow that you should have learned about variables, loops, and
conditional statements it is time to learn about functions. You should
have an idea of their uses as we have already used them and defined one
in the guise of main. Getchar is another example of a function. In
general, functions are blocks of code that perform a number of
pre-defined commands to accomplish something productive. You can either
use the built-in library functions or you can create your own
functions.
Functions that a programmer writes will generally require a prototype.
Just like a blueprint, the prototype gives basic structural
information: it tells the compiler what the function will return, what
the function will be called, as well as what arguments the function can
be passed. When I say that the function returns a value, I mean that
the function can be used in the same manner as a variable would be. For
example, a variable can be set equal to a function that returns a value
between zero and four.
For example: #include
int a = rand(); /* rand is a standard function that all compilers have
*/
Do not think that 'a' will change at random, it will be set to the
value returned when the function is called, but it will not change
again.
The general format for a prototype is simple: return-type function_name ( arg_type arg1, ..., arg_type argN );
arg_type just means the type for each argument -- for instance, an int,
a float, or a char. It's exactly the same thing as what you would put
if you were declaring a variable.
There can be more than one argument passed to a function or none at all
(where the parentheses are empty), and it does not have to return a
value. Functions that do not return values have a return type of void.
Let's look at a function prototype: int mult ( int x, int y );
This prototype specifies that the function mult will accept two
arguments, both integers, and that it will return an integer. Do not
forget the trailing semi-colon. Without it, the compiler will probably
think that you are trying to write the actual definition of the
function.
When the programmer actually defines the function, it will begin with
the prototype, minus the semi-colon. Then there should always be a
block (surrounded by curly braces) with the code that the function is
to execute, just as you would write it for the main function. Any of
the arguments passed to the function can be used as if they were
declared in the block. Finally, end it all with a cherry and a closing
brace. Okay, maybe not a cherry.
Let's look at an example program: #include
int mult ( int x, int y );
int main(){ int x; int y; printf( "Please input two numbers to be multiplied: " ); scanf( "%d", &x ); scanf( "%d", &y ); printf( "The product of your two numbers is %d\n", mult( x, y ) ); getchar(); }
int mult (int x, int y){ return x * y;}
This program begins with the only necessary include file. Next is the
prototype of the function. Notice that it has the final semi-colon! The
main function returns an integer, which you should always have to
conform to the standard. You should not have trouble understanding the
input and output functions if you've followed the previous tutorials.
Notice how printf actually takes the value of what appears to be the
mult function. What is really happening is printf is accepting the
value returned by mult, not mult itself. The result would be the same
as if we had use this print instead printf( "The product of your two numbers is %d\n", x * y );
The mult function is actually defined below main. Because its prototype
is above main, the compiler still recognizes it as being declared, and
so the compiler will not give an error about mult being undeclared. As
long as the prototype is present, a function can be used even if there
is no definition. However, the code cannot be run without a definition
even though it will compile.
Prototypes are declarations of the function, but they are only
necessary to alert the compiler about the existence of a function if we
don't want to go ahead and fully define the function. If mult were
defined before it is used, we could do away with the prototype--the
definition basically acts as a prototype as well.
Return is the keyword used to force the function to return a value.
Note that it is possible to have a function that returns no value. If a
function returns void, the retun statement is valid, but only if it
does not have an expression. In otherwords, for a function that returns
void, the statement "return;" is legal, but usually redundant. (It can
be used to exit the function before the end of the function.)
The most important functional (pun semi-intended) question is why do we
need a function? Functions have many uses. For example, a programmer
may have a block of code that he has repeated forty times throughout
the program. A function to execute that code would save a great deal of
space, and it would also make the program more readable. Also, having
only one copy of the code makes it easier to make changes. Would you
rather make forty little changes scattered all throughout a potentially
large program, or one change to the function body? So would I.
Another reason for functions is to break down a complex program into
logical parts. For example, take a menu program that runs complex code
when a menu choice is selected. The program would probably best be
served by making functions for each of the actual menu choices, and
then breaking down the complex tasks into smaller, more manageable
tasks, which could be in their own functions. In this way, a program
can be designed that makes sense when read. And has a structure that is
easier to understand quickly. The worst programs usually only have the
required function, main, and fill it with pages of jumbled code.
Lesson 5: switch caseSwitch case statements are a substitute for long if statements that
compare a variable to several "integral" values ("integral" values are
simply values that can be expressed as an integer, such as the value of
a char). The basic format for using switch case is outlined below. The
value of the variable given into switch is compared to the value
following each of the cases, and when one value matches the value of
the variable, the computer continues executing the program from that
point
switch (
of the cases break;}
The condition of a switch statement is a value. The case says that if
it has the value of whatever is after that case then do whatever
follows the colon. The break is used to break out of the case
statements. Break is a keyword that breaks out of the code block,
usually surrounded by braces, which it is in. In this case, break
prevents the program from falling through and executing the code in all
the other case statements. An important thing to note about the switch
statement is that the case values may only be constant integral
expressions. Sadly, it isn't legal to use case like this: int a = 10;int b = 10;int c = 20;
switch ( a ) {case b: /* Code */ break;case c: /* Code */ break;default: /* Code */ break;}
The default case is optional, but it is wise to include it as it
handles any unexpected cases. It can be useful to put some kind of
output to alert you to the code entering the default case if you don't
expect it to. Switch statements serve as a simple way to write long if
statements when the requirements are met. Often it can be used to
process input from a user.
Below is a sample program, in which not all of the proper functions are
actually declared, but which shows how one would use switch in a
program. #include
void playgame();void loadgame();void playmultiplayer(); int main(){ int input;
printf( "1. Play game\n" ); printf( "2. Load game\n" ); printf( "3. Play multiplayer\n" ); printf( "4. Exit\n" ); printf( "Selection: " ); scanf( "%d", &input ); switch ( input ) { case 1: /* Note the colon, not a semicolon */ playgame(); break; case 2: loadgame(); break; case 3: playmultiplayer(); break; case 4: printf( "Thanks for playing!\n" ); break; default: printf( "Bad input, quitting!\n" ); break; } getchar();
}
This program will compile, but cannot be run until the undefined
functions are given bodies, but it serves as a model (albeit simple)
for processing input. If you do not understand this then try mentally
putting in if statements for the case statements. Default simply skips
out of the switch case construction and allows the program to terminate
naturally. If you do not like that, then you can make a loop around the
whole thing to have it wait for valid input. You could easily make a
few small functions if you wish to test the code.
Lesson 6: An introduction to pointers
Pointers are an extremely powerful programming tool. They can make some
things much easier, help improve your program's efficiency, and even
allow you to handle unlimited amounts of data. For example, using
pointers is one way to have a function modify a variable passed to it.
It is also possible to use pointers to dynamically allocate memory,
which means that you can write programs that can handle nearly
unlimited amounts of data on the fly--you don't need to know, when you
write the program, how much memory you need. Wow, that's kind of cool.
Actually, it's very cool, as we'll see in some of the next tutorials.
For now, let's just get a basic handle on what pointers are and how you
use them.
What are pointers? Why should you care?Pointers are aptly name: they "point" to locations in memory. Think of
a row of safety deposity boxes of various sizes at a local bank. Each
safety deposity box will have a number associated with it so that you
can quickly look it up. These numbers are like the memory addresses of
variables. A pointer in the world of safety deposit box would simply be
anything that stored the number of another safety deposit box. Perhaps
you have a rich uncle who stored valuables in his safety deposit box,
but decided to put the real location in another, smaller, safety
deposit box that only stored a card with the number of the large box
with the real jewelery. The safety deposit box with the card would be
storing the location of another box; it would be equivalent to a
pointer. In the computer, pointers are just variables that store memory
addresses, usually the addresses of other variables.
The cool thing is that once can talk about the address of a variable,
you'll then be able to go to that address and retrieve the data stored
in it. If you happen to have a huge piece of data that you want to pass
into a function, it's a lot easier to pass its location to the function
that to copy every element of the data! Moreover, if you need more
memory for your program, you can request more memory from the
system--how do you get "back" that memory? The system tells you where
it is located in memory; that is to say, you get a memory address back.
And you need pointers to store the memory address.
A note about terms: the word pointer can refer either to a memory
address itself, or to a variable that stores a memory address. Usually,
the distinction isn't really that important: if you pass a pointer
variable into a function, you're passing the value stored in the
pointer--the memory address. When I want to talk about a memory
address, I'll refer to it as a memory address; when I want a variable
that stores a memory address, I'll call it a pointer. When a variable
stores the address of another variable, I'll say that it is "pointing
to" that variable. Pointer SyntaxPointers require a bit of new syntax because when you have a pointer,
you need the ability to both request the memory location it stores and
the value stored at that memory location. Moreover, since pointers are
somewhat special, you need to tell the compiler when you declare your
pointer variable that the variable is a pointer, and tell the compiler
what type of memory it points to.
The pointer declaration looks like this:
For example, you could declare a pointer that stores the address of an
integer with the following syntax: int *points_to_integer;
Notice the use of the *. This is the key to declaring a pointer; if you
add it directly before the variable name, it will declare the variable
to be a pointer. Minor gotcha: if you declare multiple pointers on the
same line, you must precede each of them with an asterisk: /* one pointer, one regular int */int *pointer1, nonpointer1;
/* two pointers */int *pointer1, *pointer2;
As I mentioned, there are two ways to use the pointer to access
information: it is possible to have it give the actual address to
another variable. To do so, simply use the name of the pointer without
the *. However, to access the actual memory location, use the *. The
technical name for this doing this is dereferencing the pointer; in
essence, you're taking the reference to some memory address and
following it, to retrieve the actual value. It can be tricky to keep
track of when you should add the asterisk. Remember that the pointer's
natural use is to store a memory address; so when you use the pointer: call_to_function_expecting_memory_address(pointer);
then it evaluates to the address. You have to add something extra, the
asterisk, in order to retrieve the value stored at the address. You'll
probably do that an awful lot. Nevertheless, the pointer itself is
supposed to store an address, so when you use the bare pointer, you get
that address back. Pointing to Something: Retrieving an AddressIn order to have a pointer actually point to another variable it is
necessary to have the memory address of that variable also. To get the
memory address of a variable (its location in memory), put the & sign
in front of the variable name. This makes it give its address. This is
called the address-of operator, because it returns the memory address.
Conveniently, both ampersand and address-of start with a; that's a
useful way to remember that you use & to get the address of a variable.
For example: #include
int main(){ int x; /* A normal integer*/ int *p; /* A pointer to an integer ("*p" is an integer,
so p must be a pointer to an integer) */
p = &x; /* Read it, "assign the address of x to p" */ scanf( "%d", &x ); /* Put a value in x, we could also use
p here */ printf( "%d\n", *p ); /* Note the use of the * to get the value */ getchar();}
The printf outputs the value stored in x. Why is that? Well, let's look
at the code. The integer is called x. A pointer to an integer is then
defined as p. Then it stores the memory location of x in pointer by
using the address operator (&) to get the address of the variable.
Using the ampersand is a bit like looking at the label on the safety
deposit box to see its number rather than looking inside the box, to
get what it stores. The user then inputs a number that is stored in the
variable x; remember, this is the same location that is pointed to by
p. In fact, since we use an ampersand to pass the value to scanf, it
should be clear that scanf is putting the value in the address pointed
to by p. (In fact, scanf works becuase of pointers!)
The next line then passes *p into printf. *p performs the
"dereferencing" operation on p; it looks at the address stored in p,
and goes to that address and returns the value. This is akin to looking
inside a safety deposit box only to find the number of (and,
presumably, the key to ) another box, which you then open.
Notice that in the above example, the pointer is initialized to point
to a specific memory address before it is used. If this was not the
case, it could be pointing to anything. This can lead to extremely
unpleasant consequences to the program. For instance, the operating
system will probably prevent you from accessing memory that it knows
your program doesn't own: this will cause your program to crash. If it
let you use the memory, you could mess with the memory of any running
program--for instance, if you had a document opened in Word, you could
change the text! Fortunately, Windows and other modern operating
systems will stop you from accessing that memory and cause your program
to crash. To avoid crashing your program, you should always initialize
pointers before you use them.
It is also possible to initialize pointers using free memory. This
allows dynamic allocation of memory. It is useful for setting up
structures such as linked lists or data trees where you don't know
exactly how much memory will be needed at compile time, so you have to
get memory during the program's execution. We'll look at these
structures later, but for now, we'll simply examine how to request
memory from and return memory to the operating system.
The function malloc, residing in the stdlib.h header file, is used to
initialize pointers with memory from free store (a section of memory
available to all programs). malloc works just like any other function
call. The argument to malloc is the amount of memory requested (in
bytes), and malloc gets a block of memory of that size and then returns
a pointer to the block of memory allocated.
Since different variable types have different memory requirements, we
need to get a size for the amount of memory malloc should return. So we
need to know how to get the size of different variable types. This can
be done using the keyword sizeof, which takes an expression and returns
its size. For example, sizeof(int) would return the number of bytes
required to store an integer. #include
int *ptr = malloc( sizeof(int) );
This code set ptr to point to a memory address of size int. The memory
that is pointed to becomes unavailable to other programs. This means
that the careful coder should free this memory at the end of its usage
lest the memory be lost to the operating system for the duration of the
program (this is often called a memory leak because the program is not
keeping track of all of its memory).
Note that it is slightly cleaner to write malloc statements by taking
the size of the variable pointed to by using the pointer directly: int *ptr = malloc( sizeof(*ptr) );
What's going on here? sizeof(*ptr) will evaluate the size of whatever
we would get back from dereferencing ptr; since ptr is a pointer to an
int, *ptr would give us an int, so sizeof(*ptr) will return the size of
an integer. So why do this? Well, if we change ptr to point to
something else like a float, then we don't have to go back and correct
the malloc call to use sizeof(float). Since ptr would be pointing to a
float, *ptr would be a float, so sizeof(*ptr) would still give the
right size!
The free function returns memory to the operating system. free( ptr );
After freeing a pointer, it is a good idea to reset it to point to 0.
When 0 is assigned to a pointer, the pointer becomes a null pointer, in
other words, it points to nothing. By doing this, when you do something
foolish with the pointer (it happens a lot, even with experienced
programmers), you find out immediately instead of later, when you have
done considerable damage.
The concept of the null pointer is frequently used as a way of
indicating a problem--for instance, malloc returns 0 when it cannot
correctly allocate memory. You want to be sure to handle this
correctly--sometimes your operating system might actually run out of
memory and give you this value! Taking Stock of PointersPointers may feel like a very confusing topic at first but I think
anyone can come to appreciate and understand them. If you didn't feel
like you absorbed everything about them, just take a few deep breaths
and re-read the lesson. You shouldn't feel like you've fully grasped
every nuance of when and why you need to use pointers, though you
should have some idea of some of their basic uses.
Lesson 7: Structures
When programming, it is often convenient to have a single name with
which to refer to a group of a related values. Structures provide a way
of storing many different values in variables of potentially different
types under the same name. This makes it a more modular program, which
is easier to modify because its design makes things more compact.
Structs are generally useful whenever a lot of data needs to be grouped
together--for instance, they can be used to hold records from a
database or to store information about contacts in an address book. In
the contacts example, a struct could be used that would hold all of the
information about a single contact--name, address, phone number, and so
forth.
The format for defining a structure is struct Tag { Members};
Where Tag is the name of the entire type of structure and Members are
the variables within the struct. To actually create a single structure
the syntax is struct Tag name_of_single_structure;
To access a variable of the structure it goes name_of_single_structure.name_of_variable;
For example: struct example { int x;};struct example an_example; /* Treating it like a normal variable type except with the addition of struct*/an_example.x = 33; /*How to access its members */
Here is an example program: struct database { int id_number; int age; float salary;};
int main(){ struct database employee; /* There is now an employee variable that
has modifiable variables inside it.*/ employee.age = 22; employee.id_number = 1; employee.salary = 12000.21;}
The struct database declares that it has three variables in it, age,
id_number, and salary. You can use database like a variable type like
int. You can create an employee with the database type as I did above.
Then, to modify it you call everything with the 'employee.' in front of
it. You can also return structures from functions by defining their
return type as a structure type. For instance: struct database fn();
I will talk only a little bit about unions as well. Unions are like
structures except that all the variables share the same memory. When a
union is declared the compiler allocates enough memory for the largest
data-type in the union. Its like a giant storage chest where you can
store one large item, or a small item, but never the both at the same
time.
The '.' operator is used to access different variables inside a union
also.
As a final note, if you wish to have a pointer to a structure, to
actually access the information stored inside the structure that is
pointed to, you use the -> operator in place of the . operator. All
points about pointers still apply.
A quick example: #include
struct xampl { int x;};
int main(){ struct xampl structure; struct xampl *ptr;
structure.x = 12; ptr = &structure; /* Yes, you need the & when dealing with structures and using pointers to them*/ printf( "%d\n", ptr->x ); /* The -> acts somewhat like the * when does when it is used with pointers It says, get whatever is at that
memory address Not "get what that memory
address is"*/ getchar();}
Lesson 8: Arrays
Arrays are useful critters that often show up when it would be
convenient to have one name for a group of variables of the same type
that can be accessed by a numerical index. For example, a tic-tac-toe
board can be held in an array and each element of the tic-tac-toe board
can easily be accessed by its position (the upper left might be
position 0 and the lower right position 8). At heart, arrays are
essentially a way to store many values under the same name. You can
make an array out of any data-type including structures and classes.
One way to visualize an array is like this: [][][][][][]
Each of the bracket pairs is a slot in the array, and you can store
information in slot--the information stored in the array is called an
element of the array. It is very much as though you have a group of
variables lined up side by side.
Let's look at the syntax for declaring an array. int examplearray[100]; /* This declares an array */
This would make an integer array with 100 slots (the places in which
values of an array are stored). To access a specific part element of
the array, you merely put the array name and, in brackets, an index
number. This corresponds to a specific element of the array. The one
trick is that the first index number, and thus the first element, is
zero, and the last is the number of elements minus one. The indices for
a 100 element array range from 0 to 99. Be careful not to "walk off the
end" of the array by trying to access element 100!
What can you do with this simple knowledge? Lets say you want to store
a string, because C has no built-in datatype for strings, you can make
an array of characters.
For example: char astring[100];
will allow you to declare a char array of 100 elements, or slots. Then
you can receive input into it from the user, and when the user types in
a string, it will go in the array, the first character of the string
will be at position 0, the second character at position 1, and so
forth. It is relatvely easy to work with strings in this way because it
allows support for any size string you can imagine all stored in a
single variable with each element in the string stored in an adjacent
location--think about how hard it would be to store nearly arbitrary
sized strings using simple variables that only store one value. Since
we can write loops that increment integers, it's very easy to scan
through a string: char astring[10];int i = 0;/* Using scanf isn't really the best way to do this; we'll talk about
that in the next tutorial, on strings */scanf( "%s", astring );for ( i = 0; i < 10; ++i ){ if ( astring[i] == 'a' ) { printf( "You entered an a!\n" ); }}
Let's look at something new here: the scanf function call is a tad
different from what we've seen before. First of all, the format string
is '%s' instead of '%d'; this just tells scanf to read in a string
instead of an integer. Second, we don't use the ampersand! It turns out
that when we pass arrays into functions, the compiler automatically
converts the array into a pointer to the first element of the array. In
short, the array without any brackets will act like a pointer. So we
just pass the array directly into scanf without using the ampersand and
it works perfectly.
Also, notice that to access the element of the array, we just use the
brackets and put in the index whose value interests us; in this case,
we go from 0 to 9, checking each element to see if it's equal to the
character a. Note that some of these values may actually be
uninitialized since the user might not input a string that fills the
whole array--we'll look into how strings are handled in more detail in
the next tutorial; for now, the key is simply to understand the power
of accessing the array using a numerical index. Imagine how you would
write that if you didn't have access to arrays! Oh boy.
Multidimensional arrays are arrays that have more than one index:
instead of being just a single line of slots, multidimensional arrays
can be thought of as having values that spread across two or more
dimensions. Here's an easy way to visualize a two-dimensional array: [][][][][][][][][][][][][][][][][][][][][][][][][]
The syntax used to actually declare a two dimensional array is almost
the same as that used for declaring a one-dimensional array, except
that you include a set of brackets for each dimension, and include the
size of the dimension. For example, here is an array that is large
enough to hold a standard checkers board, with 8 rows and 8 columns: int two_dimensional_array[8][8];
You can easily use this to store information about some kind of game or
to write something like tic-tac-toe. To access it, all you need are two
variables, one that goes in the first slot and one that goes in the
second slot. You can make three dimensional, four dimensional, or even
higher dimensional arrays, though past three dimensions, it becomes
quite hard to visualize.
Setting the value of an array element is as easy as accessing the
element and performing an assignment. For instance,
for instance, /* set the first element of my_first to be the letter c */my_string[0] = 'c';
or, for two dimensional arrays
Let me note again that you should never attempt to write data past the
last element of the array, such as when you have a 10 element array,
and you try to write to the [10] element. The memory for the array that
was allocated for it will only be ten locations in memory, (the
elements 0 through 9) but the next location could be anything. Writing
to random memory could cause unpredictable effects--for example you
might end up writing to the video buffer and change the video display,
or you might write to memory being used by an open document and
altering its contents. Usually, the operating system will not allow
this kind of reckless behavior and will crash the program if it tries
to write to unallocated memory.
You will find lots of useful things to do with arrays, from storing
information about certain things under one name, to making games like
tic-tac-toe. We've already seen one example of using loops to access
arrays; here is another, more interesting, example! #include
int main(){ int x; int y; int array[8][8]; /* Declares an array like a chessboard */ for ( x = 0; x < 8; x++ ) { for ( y = 0; y < 8; y++ ) array[x][y] = x * y; /* Set each element to a value */ } printf( "Array Indices:\n" ); for ( x = 0; x < 8;x++ ) { for ( y = 0; y < 8; y++ ) { printf( "[%d][%d]=%d", x, y, array[x][y] ); } printf( "\n" ); } getchar();}
Just to touch upon a final point made briefly above: arrays don't
require a reference operator (the ampersand) when you want to have a
pointer to them. For example: char *ptr;char str[40];ptr = str; /* Gives the memory address without a reference operator(&)
*/
As opposed to int *ptr;int num;ptr = # /* Requires & to give the memory address to the ptr */
The fact that arrays can act just like pointers can cause a great deal
of confusion. For more information please see our Frequently Asked
Questions.
Lesson 9: C StringsThis lesson will discuss C-style strings, which you may have already
seen in the array tutorial. In fact, C-style strings are really arrays
of chars with a little bit of special sauce to indicate where the
string ends. This tutorial will cover some of the tools available for
working with strings--things like copying them, concatenating them, and
getting their length. What is a String?Note that along with C-style strings, which are arrays, there are also
string literals, such as "this". In reality, both of these string types
are merely just collections of characters sitting next to each other in
memory. The only difference is that you cannot modify string literals,
whereas you can modify arrays. Functions that take a C-style string
will be just as happy to accept string literals unless they modify the
string (in which case your program will crash). Some things that might
look like strings are not strings; in particular, a character inclosed
in single quotes, like this, 'a', is not a string. It's a single
character, which can be assigned to a specific location in a string,
but which cannot be treated as a string. (Remember how arrays act like
pointers when passed into functions? Characters don't, so if you pass a
single character into a function, it won't work; the function is
expecting a char*, not a char.)
To recap: strings are arrays of chars. String literals are words
surrounded by double quotation marks. "This is a static string"
Remember that special sauce mentioned above? Well, it turns out that
C-style strings are always terminated with a null character, literally
a '\0' character (with the value of 0), so to declare a string of 49
letters, you need to account for it by adding an extra character, so
you would want to say: char string[50];
This would declare a string with a length of 50 characters. Do not
forget that arrays begin at zero, not 1 for the index number. In
addition, we've accounted for the extra with a null character,
literally a '\0' character. It's important to remember that there will
be an extra character on the end on a string, just like there is always
a period at the end of a sentence. Since this string terminator is
unprintable, it is not counted as a letter, but it still takes up a
space. Technically, in a fifty char array you could only hold 49
letters and one null character at the end to terminate the string.
Note that something like char *my_string;
can also be used as a string. If you have read the tutorial on
pointers, you can do something such as: arry = new char[256];
which allows you to access arry just as if it were an array. Keep in
mind that to use delete you must put [] between delete and arry to tell
it to free all 256 bytes of memory allocated.
For example: delete [] arry.
Using StringsStrings are useful for holding all types of long input. If you want the
user to input his or her name, you must use a string. Using scanf() to
input a string works, but it will terminate the string after it reads
the first space, and moreover, because scanf doesn't know how big the
array is, it can lead to "buffer overflows" when the user inputs a
string that is longer than the size of the string (which acts as an
input "buffer").
There are several approaches to handling this problem, but probably the
simplest and safest is to use the fgets function, which is declared in
stdio.h.
The prototype for the fegets function is: char *fgets (char *str, int size, FILE* file);
There are a few new things here. First of all, let's clear up the
questions about that funky FILE* pointer. The reason this exists is
because fgets is supposed to be able to read from any file on disk, not
just from the user's keyboard (or other "standard input" device). For
the time being, whenever we call fgets, we'll just pass in a variable
called stdin, defined in stdio.h, which refers to "standard input".
This effectively tells the program to read from the keyboard. The other
two arguments to fgets, str and size, are simply the place to store the
data read from the input and the size of the char*, str. Finally, fgets
returns str whenever it successfully read from the input.
When fgets actually reads input from the user, it will read up to size
- 1 characters and then place the null terminator after the last
character it read. fgets will read input until it either has no more
room to store the data or until the user hits enter. Notice that fgets
may fill up the entire space allocated for str, but it will never
return a non-null terminated string to you.
Let's look at an example of using fgets, and then we'll talk about some
pitfalls to watch out for.
For a example: #include
int main(){ /* A nice long string */ char string[256];
printf( "Please enter a long string: " );
/* notice stdin being passed in */ fgets ( string, 256, stdin );
printf( "You entered a very long string, %s", string );
getchar();}
Remember that you are actually passing the address of the array when
you pass string because arrays do not require an address operator (&)
to be used to pass their addresses, so the values in the array string
are modified.
The one thing to watch out for when using fgets is that it will include
the newline character ('\n') when it reads input unless there isn't
room in the string to store it. This means that you may need to
manually remove the input. One way to do this would be to search the
string for a newline and then replace it with the null terminator. What
would this look like? See if you can figure out a way to do it before
looking below: char input[256];int i;
fgets( input, 256, stdin );
for ( i = 0; i < 256; i++ ){ if ( input[i] == '\n' ) { input[i] = '\0'; break; }}
Here, we just loop through the input until we come to a newline, and
when we do, we replace it with the null terminator. Notice that if the
input is less than 256 characters long, the user must have hit enter,
which would have included the newline character in the string! (By the
way, aside from this example, there are other approaches to solving
this problem that use functions from string.h.) Manipulating C strings using string.hstring.h is a header file that contains many functions for manipulating
strings. One of these is the string comparison function. int strcmp ( const char *s1, const char *s2 );
strcmp will accept two strings. It will return an integer. This integer
will either be: Negative if s1 is less than s2.Zero if s1 and s2 are equal.Positive if s1 is greater than s2.
Strcmp performs a case sensitive comparison; if the strings are the
same except for a difference in cAse, then they're countered as being
different. Strcmp also passes the address of the character array to the
function to allow it to be accessed. char *strcat ( char *dest, const char *src );
strcat is short for "string concatenate"; concatenate is a fancy word
that means to add to the end, or append. It adds the second string to
the first string. It returns a pointer to the concatenated string.
Beware this function; it assumes that dest is large enough to hold the
entire contents of src as well as its own contents. char *strcpy ( char *dest, const char *src );
strcpy is short for string copy, which means it copies the entire
contents of src into dest. The contents of dest after strcpy will be
exactly the same as src such that strcmp ( dest, src ) will return 0. size_t strlen ( const char *s );
strlen will return the length of a string, minus the termating
character ('\0'). The size_t is nothing to worry about. Just treat it
as an integer that cannot be negative, which is what it actually is.
(The type size_t is just a way to indicate that the value is intended
for use as a size of something.)
Here is a small program using many of the previously described
functions: #include
/* this function is designed to remove the newline from the end of a
stringentered using fgets. Note that since we make this into its own
function, wecould easily choose a better technique for removing the newline.
Aren'tfunctions great? */void strip_newline( char *str, int size ){ int i;
/* remove the null terminator */ for ( i = 0; i < size; ++i ) { if ( str[i] == '\n' ) { str[i] = '\0';
/* we're done, so just exit the function by returning */ return; } } /* if we get all the way to here, there must not have been a
newline! */}
int main(){ char name[50]; char lastname[50]; char fullname[100]; /* Big enough to hold both name and lastname */
printf( "Please enter your name: " ); fgets( name, 50, stdin );
/* see definition above */ strip_newline( name, 50 );
/* strcmp returns zero when the two strings are equal */ if ( strcmp ( name, "Alex" ) == 0 ) { printf( "That's my name too.\n" ); } else { printf( "That's not my name.\n" ); } // Find the length of your name printf( "Your name is %d letters long", strlen ( name ) ); printf( "Enter your last name: " ); fgets( lastname, 50, stdin ); strip_newline( lastname, 50 ); fullname[0] = '\0'; /* strcat will look for the \0 and add the second string starting
at that location */ strcat( fullname, name ); /* Copy name into full name */ strcat( fullname, " " ); /* Separate the names by a space */ strcat( fullname, lastname ); /* Copy lastname onto the end of
fullname */ printf( "Your full name is %s\n",fullname );
getchar();
return 0;}
Safe ProgrammingThe above string functions all rely on the existence of a null
terminator at the end of a string. This isn't always a safe bet.
Moreover, some of them, noticeably strcat, rely on the fact that the
destination string can hold the entire string being appended onto the
end. Although it might seem like you'll never make that sort of
mistake, historically, problems based on accidentally writing off the
end of an array in a function like strcat, have been a major problem.
Fortunately, in their infinite wisdom, the designers of C have included
functions designed to help you avoid these issues. Similar to the way
that fgets takes the maximum number of characters that fit into the
buffer, there are string functions that take an additional argument to
indicate the length of the destination buffer. For instance, the strcpy
function has an analogous strncpy function char *strncpy ( char *dest, const char *src, size_t len );
which will only copy len bytes from src to dest (len should be less
than the size of dest or the write could still go beyond the bounds of
the array). Unfortunately, strncpy can lead to one niggling issue: it
doesn't guarantee that dest will have a null terminator attached to it
(this might happen if the string src is longer than dest). You can
avoid this problem by using strlen to get the length of src and make
sure it will fit in dest. Of course, if you were going to do that, then
you probably don't need strncpy in the first place, right? Wrong. Now
it forces you to pay attention to this issue, which is a big part of
the battle.
C File I/O and Binary File I/O
When accessing files through C, the first necessity is to have a way to
access the files. For C File I/O you need to use a FILE pointer, which
will let the program keep track of the file being accessed. (You can
think of it as the memory address of the file or the location of the
file).
For example:
FILE *fp;To open a file you need to use the fopen function, which returns a FILE
pointer. Once you've opened a file, you can use the FILE pointer to let
the compiler perform input and output functions on the file.
FILE *fopen(const char *filename, const char *mode);In the filename, if you use a string literal as the argument, you need
to remember to use double backslashes rather than a single backslash as
you otherwise risk an escape character such as \t. Using double
backslashes \\ escapes the \ key, so the string works as it is
expected. Your users, of course, do not need to do this! It's just the
way quoted strings are handled in C and C++.
The modes are as follows:
r - open for readingw - open for writing (file need not exist)a - open for appending (file need not exist)r+ - open for reading and writing, start at beginningw+ - open for reading and writing (overwrite file)a+ - open for reading and writing (append if file exists)Note that it's possible for fopen to fail even if your program is
perfectly correct: you might try to open a file specified by the user,
and that file might not exist (or it might be write-protected). In
those cases, fopen will return 0, the NULL pointer.
Here's a simple example of using fopen:
FILE *fp;fp=fopen("c:\\test.txt", "r");This code will open test.txt for reading in text mode. To open a file
in a binary mode you must add a b to the end of the mode string; for
example, "rb" (for the reading and writing modes, you can add the b
either after the plus sign - "r+b" - or before - "rb+")
To close a function you can use the function
int fclose(FILE *a_file);fclose returns zero if the file is closed successfully.
An example of fclose is fclose(fp);To work with text input and output, you use fprintf and fscanf, both of
which are similar to their friends printf and scanf except that you
must pass the FILE pointer as first argument. For example:
FILE *fp;fp=fopen("c:\\test.txt", "w");fprintf(fp, "Testing...\n");It is also possible to read (or write) a single character at a
time--this can be useful if you wish to perform character-by-character
input (for instance, if you need to keep track of every piece of
punctuation in a file it would make more sense to read in a single
character than to read in a string at a time.) The fgetc function,
which takes a file pointer, and returns an int, will let you read a
single character from a file: int fgetc (FILE *fp);
Notice that fgetc returns an int. What this actually means is that when
it reads a normal character in the file, it will return a value
suitable for storing in an unsigned char (basically, a number in the
range 0 to 255). On the other hand, when you're at the very end of the
file, you can't get a character value--in this case, fgetc will return
"EOF", which is a constnat that indicates that you've reached the end
of the file. To see a full example using fgetc in practice, take a look
at the example here.
The fputc function allows you to write a character at a time--you might
find this useful if you wanted to copy a file character by character.
It looks like this: int fputc( int c, FILE *fp );
Note that the first argument should be in the range of an unsigned char
so that it is a valid character. The second argument is the file to
write to. On success, fputc will return the value c, and on failure, it
will return EOF. Binary I/OFor binary File I/O you use fread and fwrite.
The declarations for each are similar: size_t fread(void *ptr, size_t size_of_elements, size_t
number_of_elements, FILE *a_file); size_t fwrite(const void *ptr, size_t size_of_elements, size_t
number_of_elements, FILE *a_file);Both of these functions deal with blocks of memories - usually arrays.
Because they accept pointers, you can also use these functions with
other data structures; you can even write structs to a file or a read
struct into memory.
Let's look at one function to see how the notation works.
fread takes four arguments. Don't by confused by the declaration of a
void *ptr; void means that it is a pointer that can be used for any
type variable. The first argument is the name of the array or the
address of the structure you want to write to the file. The second
argument is the size of each element of the array; it is in bytes. For
example, if you have an array of characters, you would want to read it
in one byte chunks, so size_of_elements is one. You can use the sizeof
operator to get the size of the various datatypes; for example, if you
have a variable int x; you can get the size of x with sizeof(x);. This
usage works even for structs or arrays. Eg, if you have a variable of a
struct type with the name a_struct, you can use sizeof(a_struct) to
find out how much memory it is taking up.
e.g.,
sizeof(int);
The third argument is simply how many elements you want to read or
write; for example, if you pass a 100 element array, you want to read
no more than 100 elements, so you pass in 100.
The final argument is simply the file pointer we've been using. When
fread is used, after being passed an array, fread will read from the
file until it has filled the array, and it will return the number of
elements actually read. If the file, for example, is only 30 bytes, but
you try to read 100 bytes, it will return that it read 30 bytes. To
check to ensure the end of file was reached, use the feof function,
which accepts a FILE pointer and returns true if the end of the file
has been reached.
fwrite is similar in usage, except instead of reading into the memory
you write from memory into a file.
For example, FILE *fp;fp=fopen("c:\\test.bin", "wb");char x[10]="ABCDEFGHIJ";fwrite(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), fp);
Friday, October 26, 2007
C TUTORIALS
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